The myriad small electronic devices in use today, including computers, cellular telephones, and portable game-station, have been made possible in large part by the development of semiconductor electronics technology. Semiconductors are materials that conduct electricity only under certain conditions, which often include the presence of a small electrical charge. This enables the manufacture of solid-state switches—those that have no moving parts. Unlike former technology that relied on electromechanical switches and wires, integrated circuits made of semiconducting materials have no moving parts, take up far less space, and can operate on much less power—all desirable features for components used in modern electronic devices.
Because of their small size, semiconductor devices require a specialized fabrication process. A series of steps is used to simultaneously form a large number of components on a single work piece. The specific steps involved in fabricating a particular combination of these tiny semiconductor devices may vary, but similar general steps are employed. A material such as silicon is produced for use as a base, or substrate material. This material is then cut into an appropriate shape, typically a thin slice called a wafer. The pure silicon is then selectively treated with one or more materials called dopants, such as ionized boron or phosphorus. The introduction of these impurities begins the process of creating the desired semiconductive properties. Various structures are then formed at or near a surface of the wafer in a series of steps. These structures that will eventually make up the transistors, capacitors, and other electrical components of the particular semiconductor device.
These surface structures may be formed for example by etching, whereby the surface is exposed to an appropriate etching agent. Or, more typically, the surface is selectively etched using a process known as photolithography. In photolithography, a material called photoresist, or simply resist, is deposited and spread evenly over the wafer surface. The resist is then selectively treated with a light source directed through a patterned mask so that some portions of the resist are exposed to the light energy and others are not. The exposed portions of the photoresist are either strengthened or weakened, depending on the type of resist material used, so that the weaker portions can be washed away using a solvent that will not otherwise affect the wafer or any structures already formed on it. The resist that remains, however, will prevent the etching of the wafer surface in the still-covered areas when the etching agent is used in subsequent steps. When the desired wafer etching has been accomplished, the remaining photoresist is removed using an appropriate solvent. Etching is only one method of removing material, however, and other methods including mechanical scrubbing are also used.
Materials are added to the wafer during the fabrication process as well. Metals, other conductors, and insulators are added to the wafer surface using any of a variety of deposition methods, for example chemical vapor deposition (CVD) or sputtering. Additional ion implantation may also be performed. By selectively adding and removing these various materials, layer after layer of electrical devices can be formed on the wafer surface (or on top of previously formed structures).
A single wafer is typically used for the fabrication of a number of dice, or individual portions of the wafer that will eventually be cut apart from each other and used separately. Typically, all of the dice on a single wafer are formed identically, but this is not necessarily the case. After the fabrication is complete (and often at various intermediate steps as well), the wafer is inspected so that defective regions can be marked for discard or repair. When the dice are separated, those passing inspection are packaged, that is encapsulated in a hard plastic material and provided with external leads connected to various internal locations. The encapsulated die that has been provided with a number of leads is often referred to as a chip.
There are many types of electrical devices that can be fabricated using methods like those described above, and a single chip contains thousands, or even millions of such devices, arranged to form functioning circuits. At times it is necessary to connect one circuit or device with another one that is not immediately adjacent to it. An interconnect may be used for this purpose. A typical interconnect structure of this prior art is shown in FIGS. 1A and 1B. FIG. 1A is a cross-sectional side view of semiconductor device 100. FIG. 1B is a cross-sectional side view of semiconductor device 100, rotated 90° about a vertical axis from the view of FIG. 1A. Semiconductor device 100, formed on substrate 101, includes a first electrical circuit and a second electrical circuit, the two of which need to be connected for the device to operate. The specific function of these circuits is not relevant here, and for convenience they will be referred to as first active area 105 and second active area 110. These active areas are formed on substrate 101 in a first layer 102. Note that as used herein, the term “semiconductor device” may refer to a complete device that will function as a unit, but it will frequently also be used to refer to a component or collection of components that will make up only a small portion of such a functional unit.
An elongated conductor 115 formed in trench 116 is used to electrically connect first active area 105 and second active area 110. Elongated conductor 115 is located in interconnect layer 103, and is constructed of a conducting material such as aluminum or copper. To make sure the elongated conductor 115 does not contact any other portions of the semiconductor device (unless it is intended to do so), a dielectric material 120 surrounds it, locally filling in much of the remainder of layer 103. (Although not shown in FIGS. 1A and 1B, an insulating layer or additional interconnect structures, or both, are likely to be disposed in an additional layer above layer 103.) To establish an electrical connection with the appropriate active areas, vias 117 and 118 are formed, extending downward from trench 116, and also filled with the conductive material of elongated conductor 115. Note that in fabricating a structure such as semiconductor device 100, the trench and the vias are typically formed in a previously-deposited dielectric material and then filled with the conductor. As illustrated in FIGS. 1A and 1B, via 117 extends to contact active area 105 and via 118 extends to contact active area 110.
The interconnect of FIGS. 1A and 1B is adequate for use in many semiconductor applications. As the drive continues to design chips containing a greater number of electrical devices in smaller and smaller packages, however, problems associated with heat dissipation, stress migration, and mechanical strength, among other factors, become more pronounced. Improvement in design to deal with these problems is constantly demanded. The present invention provides just such an improvement.